Critical Currents and Thermally Activated Flux Motion in High Temperature Superconductors.

We have measured the resistance below T sub c of single crystals of the high temperature superconductors Ba sub 2 YCu sub 3 O sub 7 and Bi sub (2.2) Sr sub 2 Ca sub (0.8) Cu sub 2 O sub (8+delta) in magnetic fields up to 12T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below T sub c. The dissipation is significantly larger and of different character in the Bi- Cu-compound than in Ba sub 2 YCu sub 3 O sub 7. The relation between the activated behavior and the depinning critical current is discussed.